1. Field of the Invention
This invention relates to a light source for an external cavity laser to be used in the field of optical communication.
2. Description of the Related Art
A light source for an external cavity laser in the related art will be described below with reference to FIGS. 8 to 11.
As shown in FIG. 8, in the light source for an external cavity laser of the related art, an antireflection coating 15A is applied onto one facet of a semiconductor laser 15. A light beam emitted from the facet of the antireflection coating 15A side is converted into a parallel light beam by a lens 5. Wavelength selection of the parallel light beam is performed by a diffraction grating 2. Subsequently, the light beam is returned to the diffraction grating 2 by a mirror 3 to perform wavelength selection again by the diffraction grating 2 and to feed back the light beam to the semiconductor laser 15 to perform laser oscillation.
With regard to the output light beam, the light beam emitted from the other facet of the semiconductor laser 15 is converted into a parallel light beam by lens 6. After passage through an optical isolator 8, the light beam is converged by lens 7 into an optical fiber 4 to be taken out.
The system illustrated in FIG. 8 is called a Rittman type, and since wavelength selection is performed twice by the diffraction grating 2 in the round-trip, it is excellent in wavelength selectivity and is presently known as the most general type of system.
With the arrangement shown in FIG. 9, a beam splitter 9, which takes out a part of the diffracted light beam that is fed back from the diffraction grating 2 to semiconductor laser 15, is equipped between the semiconductor laser 15 and the diffraction grating 2 of the light source for an external cavity laser of FIG. 8, and the diffracted light beam 10 that is taken out by beam splitter 9 is converged and output into optical fiber 11 via an optical isolator 12 and a lens 13. Since the diffracted light 10 is the light that has just undergone the wavelength selection twice by the diffraction grating 2 in the round-trip, though the optical output will be somewhat lower in comparison to the output of the optical fiber 4, an extremely pure single-wavelength light beam, which does not contain any of the spontaneous emission components that emitted from semiconductor laser 15, can be obtained (refer to Japanese Unexamined Patent Publication No.Hei.11-126943).
Meanwhile, FIG. 10 shows an example where a band-pass filter 14 is used in place of the diffraction grating of the light source for an external cavity laser of FIG. 8.
In a light source for an external cavity laser, such as those shown in FIGS. 8 to 10, though the antireflection coating 15A that is applied to one facet of the semiconductor laser 15 is essential for forming the external resonator, the other facet is not provided in particular with a coating due to reasons of cost and is left in the form of a cleavage plane (the surface as it is when the element is cleaved; the reflectivity is approximately 32%), as in a general Fabry-Perot laser.
FIGS. 11 and 12 show examples where a total reflection coating 16B, with a reflectivity of substantially 100%, is applied to one facet of a semiconductor laser 16 at the side on which an antireflection coating 16A is not applied in order to increase the optical output of the optical fiber 4 or the optical fiber 11 of the examples of the related art in FIG. 8 or 9 as much as possible.
In the example shown in FIG. 12, since the total reflection coating 16B is used at one facet of the semiconductor laser 16, the output light beam is obtained as the 0th-order light of the diffraction grating 2.
Normally in a light source for an external cavity laser, when the efficiency of the resonator drops below that of a Fabry-Perot laser diode, with which both facets are cleavage planes, the optical resonance itself weakens and the laser emission conditions tend to become unstable.
As a most simple indicator for the efficiency of the resonator, the product of the reflectivities of both facets of the resonator may be compared (in the case of an external resonator, the product of the reflectivity of the facet at one side and the feedback efficiency of the external oscillator). In a Fabry-Perot laser diode, since both facets are cleavage planes and the reflectivity of each of the facets is approximately 32% in the case of an element with an emission wavelength of 1550 nm, the product will be:
0.32xc3x970.32≈0.1 
In the case of the external resonator type semiconductor laser light source, an antireflection coating is normally applied to one facet of the semiconductor laser, and the other facet of the semiconductor laser is a cleavage plane. In the case of an element for 1550 nm, this cleavage plane is a partially reflecting surface with a reflectivity of approximately 32%. Thus in an external resonator, the semiconductor laser will not undergo emission by itself and laser emission occurs as a result of the feedback of light. The feedback efficiency thus corresponds to the reflectivity of the other facet.
In case of the example shown in FIG. 8, if the coupling efficiency of the lens 5 is 50%, the diffraction efficiency of the diffraction grating 2 is 80%, and the reflectivity of the mirror 3 is 95%, thus:
Feedback efficiency≈0.5xc3x97(0.8xc3x970.8)xc3x970.95xc3x97100=30.4% 
It is found that the feedback efficiency is thus lowered to approximately 30%.
Since the product of the reflectivities of both facets of the resonator is thus:
0.32xc3x970.304≈0.097 
It can be understood that the efficiency of the resonator is lowered and the emission condition tends to be unstable.
In the example of FIG. 8, since the light beam is made to undergo spectral separation twice by the diffraction grating in the round-trip in order to increase the wavelength selectivity, even if the diffraction efficiency is 80%, the efficiency is lowered to 64% in the round-trip. Accordingly, it is difficult to increase the feedback efficiency using the system of FIG. 8.
Furthermore, in the light source for an external cavity laser of FIG. 9, since the beam splitter 9 is provided between the semiconductor laser 15 and the diffraction grating 2, the light beam passes through the beam splitter 9 twice in addition to being diffracted by the diffraction grating 2 in the round-trip. Thus if the splitting efficiency of the beam splitter is given as 80% transmittance and 20% reflectance, the feedback efficiency is:
0.5xc3x97(0.8xc3x970.8)xc3x97(0.8xc3x970.8)xc3x970.95xc3x97100=19.5% 
Thus, the feed back efficiency is lowered to approximately 20%.
Likewise, the product of the reflectivities of the facets of the resonator is:
0.32xc3x970.195≈0.06 
Though it is desirable here to improve the coupling efficiency of the lens part, which is considered to be highest in loss, since the emission NA of the semiconductor laser takes on a large value of 0.4 to 0.5 and a cross-sectional area (emission area) of the active layer is only a few xcexcm, that is extremely small, it is difficult to achieve significant improvements even when an aspherical lens having low aberration, etc. is used.
As shown in FIG. 10, the same applies in case where band-pass filter 14 is used. That is, since a band-pass filter 14, that is narrow in half-width and excellent in wavelength selectivity, is generally high in transmission loss and the feedback efficiency thus tends to be low, it is also difficult to obtain stable oscillation conditions.
Also in the examples shown in FIGS. 11 and 12, though the optical output of the optical fiber 4 or the optical fiber 11 may be increased as much as possible by the application of the total-reflection coating 16B, since oscillation tends to occur readily even with a slight reflection at the facet on which antireflection coating 16A is applied, a composite resonator tends to be constructed, and thus it is difficult to achieve stable emission without multi-modes and mode hopping as the light source for an external cavity laser (this is especially significant in the case of a variable wavelength light source)
An object of the invention is to restrain unstable oscillation, such as multi-mode emission and mode hopping, and stabilize emission conditions in a light source for an external cavity laser even when the efficiency of feedback of light to the semiconductor laser is lowered by the loss due to various optical elements.
In order to achieve the above object, according to a first aspect of the invention, there is provided a light source for an external cavity laser, for example as shown in FIG. 1, which comprises a semiconductor laser 1, having an antireflection coating 1A provided on one end face thereof, and
the light source for an external cavity laser converts a light beam emitted from the end face at the antireflection coating 1A side of the abovementioned semiconductor laser 1 into a parallel light beams, performs wavelength selection of the light beam by means of a wavelength selection optical element 2, then returns the light beam back to the wavelength selection optical element 2 by means of a mirror 3, then feeds back the light to the abovementioned semiconductor laser 1 upon performing wavelength selection again by means of the wavelength selection optical element 2, and
converges and outputs a light beam emitted from the other end face of the abovementioned semiconductor laser 1 into an optical fiber 4,
wherein a light source for an external cavity laser is characterized in that an increased reflection coating 1B, having prescribed reflectivity characteristics, is applied onto the end face of the abovementioned semiconductor laser 1 at the side on which antireflection coating 1A is not provided.
According to the first aspect of the invention, since the light source for an external cavity laser has an increased reflection coating, which has prescribed reflectivity characteristics, applied onto the end face of the semiconductor laser at the side on which the antireflection coating is not applied, even when the efficiency of feedback to the semiconductor laser is lowered by the passage of the light via the wavelength selection optical elements twice in the round-trip, a stable light source can be provided with which the occurrence of unstable emission, such as multi-mode emission and mode hopping, is restrained.
The light source for an external cavity laser may include a diffraction grating 2 that is used as the abovementioned wavelength selection optical element.
The light source for an external cavity laser may includes a beam splitter 9, which takes out a part of the diffracted light that is fed back from the diffraction grating 2 to the abovementioned semiconductor laser 1, between semiconductor laser 1 and diffraction grating 2, wherein the diffracted light beam that is taken out by the abovementioned beam splitter 9 is converged and output into an optical fiber 11.
In the invention, since the diffracted light beam is converged and output into the optical fiber, an extremely pure single-wavelength light beam, which does not contain any spontaneous emission components, can be obtained, and since an increased reflection coating is applied, a light source of stable emission conditions can be obtained even when the efficiency of feedback to the semiconductor laser is lowered to a level less than or equal to the first aspect of the invention due to the passage of light through the beam splitter and via the diffraction grating twice in the round-trip.
The light source for an external cavity laser may be equipped with a rotation mechanism (indicated by the arrow) that enables variation of the selected wavelength by the changing of the angle of the abovementioned mirror 3.
Since a rotation mechanism is equipped that enables variation of the selected wavelength by the changing of the angle of the mirror, the merit that an arbitrary wavelength can be selected freely is provided.
In the light source for an external cavity laser, a band-pass filter 14 may be used as the abovementioned wavelength selection optical element.
A beam splitter 9, which takes out a part of the diffracted light beam that is fed back from band-pass filter 14 to the abovementioned semiconductor laser 1, may be provided between semiconductor laser 1 and band-pass filter 14 and
the diffracted light beam that is taken out by the abovementioned beam splitter 9 may be converged and output into an optical fiber 11.
Since the diffracted light beam, which is taken out by the beam splitter that takes out a part of the diffracted light that is fed back from the band-pass filter to the semiconductor laser, is converged and output into an optical fiber, an extremely pure single-wavelength light, which does not contain any spontaneous emission components, can be obtained, and since an increased reflection coating is applied, a light source of stable emission conditions can be obtained even when the efficiency of feedback to the semiconductor laser is lowered to a level less than or equal to that of the first aspect of the invention due to the passage of light through the beam splitter and the band-pass filter twice in the round-trip.
A partially reflecting mirror 17 may be used as the abovementioned mirror, and
a part of the light beam that is fed back to the abovementioned semiconductor laser may be converged and output into an optical fiber 11.
Since the mirror is made a partially reflecting mirror and a part of the light beam that is fed back to the semiconductor laser is transmitted and then converged and output into an optical fiber, an extremely pure single-wavelength light beam, which is low in spontaneous emission components, can be obtained, and since an increased reflection coating is applied, a light source of stable emission conditions can be obtained even when the efficiency of feedback to the semiconductor laser is lowered.
The abovementioned band-pass filer 14 may be a variable band-pass filter with which the film thickness is varied in a continuous manner and
a slide mechanism (indicated by the arrow) may be equipped for enabling the variation of the selected wavelength by the changing of the position of the abovementioned variable band-pass filter 14.
Since a slide mechanism is equipped for varying the selected wavelength by the changing of the position of a variable band-pass filter, with which the film thickness is varied in a continuous manner, an arbitrary wavelength can be selected freely.